1. Field of the Invention
The invention relates generally to the field of drilling wellbores through the earth. More particularly, the invention relates to methods for determining actual drilling depth of a drill string in a wellbore with respect to time, and application of the actual depth to drilling process control. The invention further relates to methods for characterizing drilling data on the basis of likely quality, and applications for the characterized data.
2. Background Art
Drilling wellbores through the earth includes “rotary” drilling, in which a drilling rig or similar lifting device suspends a drill string in the wellbore. The drill string turns a drill bit located at one end of the drill string. Equipment on the rig, and/or an hydraulically operated motor disposed in the drill string, rotate the drill bit. The rig lifting equipment is adapted to suspend the drill string so as to place a selected axial force on the drill bit as the bit is rotated. The combined axial force and bit rotation causes the bit to gouge, scrape and/or crush the rocks, thereby drilling a wellbore through the rocks. Typically a drilling rig includes liquid pumps for forcing a fluid called “drilling mud” through the interior of the drill string. The mud is ultimately discharged through nozzles or water courses in the bit. The mud lifts drill cuttings from the wellbore and carries them to the earth's surface for disposition. Other types of rigs may use compressed air as the fluid for lifting cuttings.
The drilling rig typically includes sensors for measuring drilling operating parameters. Such sensors include a “hook load” sensor, which measures the weight being suspended by the lifting equipment on the rig. By measuring the suspended weight, the amount of axial force applied to the drill bit can be inferred from the difference between the total drill string weight (which can be measured and/or calculated) and the suspended weight. The sensors also typically include a device for measuring the vertical position of the lifting equipment within the rig structure. By determining the vertical position and combining therewith a length of the drill string coupled above the drill bit, a depth in the wellbore of the drill bit (and thus the instantaneous depth of the wellbore) can be inferred. Length of the drill string can be determined by adding together the lengths of individual segments of drill pipe and a bottom hole assembly used to turn the bit. The segments and bottom hole assembly components are threadedly coupled and uncoupled by the rig equipment, as is known in the art.
Other rig sensors may include pressure gauges and volume calculators to measure pressure and volume of the mud actually pumped through the drill string. Such measurements can help the wellbore operator determine whether mud is entering the wellbore from formations being drilled, or whether mud is being lost from the wellbore into such formations, among other things.
The instantaneous depth of the wellbore is among the more important measurements made by the various sensors disposed on the drilling rig. Measurements of depth are used in determining the geologic structure of the earth formations being drilled, and there are well known methods for estimating subsurface formation fluid pressures which relate to the rate at which the formations are being drilled. One such method is known in the art as the “drilling exponent” or “d-exponent.” The d-exponent is a quantity which is determined with respect to the depth in the wellbore. The relationship between d-exponent and depth is compared to similar correlations made in nearby wellbores which have penetrated similar formations. Deviations of the d-exponent from a locally expected trend with respect to depth is an indication of unexpectedly high or low formation fluid pressures. By acting on such indications, the wellbore operator may avoid expensive and dangerous wellbore pressure control problems. Accurate determination of the d-exponent is based on accurate determination of both drilling depth and the rate at which the drilling depth changes as formations are being drilled, known as rate of penetration (“ROP”).
Another important use for instantaneous depth measurements is their ultimate correlation with measurements made by instruments coupled to the drill string, and sensors disposed at the earth's surface. Such instruments include sensors for measuring various physical properties of the formations being drilled, such as electrical conductivity, acoustic velocity, bulk density and natural gamma radiation intensity. The instruments record values related the physical properties with respect to the time of recording. At the earth's surface, a record is made of wellbore depth with respect to time. After the instruments are retrieved from the wellbore, the time-referenced recordings are correlated to the depth-time record. The result is a data set which is correlated to depth within the wellbore at which the measurements were made. As is known in the art, such depth-based records of physical properties of the formation have a number of uses, including determining geologic structures and determining presence of possible formation fluid pressure anomalies. As is the case with determining the d-exponent, determining a precise record of formation properties with respect to depth in the wellbore requires a precise determination of depth with respect to time.
Systems known in the art for determining depth with respect to time, and for determining ROP have proven less than ideal. One limitation of prior art depth measurement techniques using top drive (or kelly) vertical position measurements is that they do not account well for changes in axial length of the drill string as a result of changes in axial load on the drill string. Typically, the length of the drill string is assumed to be substantially constant. Frequently, due to sliding friction between the drill string and the wall of the wellbore, among other factors, the top drive or kelly can move a significant distance before the drill bit moves axially at all. Other methods for determining depth include a fixed correction for the axial length of the drill string. However, such methods only correct drill string length statically. In many cases, the drilling progresses at such a high rate that drill string compression (shortening) due to increases in axial force applied to the drill string does not exactly correspond to the true change in the length of the drill string Depth measurements known in the art and made only from the vertical position measurements, even when such measurements are corrected for drill string loading, are thus subject to error. ROP determination is directly related to depth measurement, and thus is correspondingly subject to error using depth measurement methods known in the art. It is therefore desirable to have a system for improving the measurement of bit depth so that more precise records of depth with respect to time, and better quality calculations based on depth may be made.
Another aspect of prior art data recording techniques is that there are not any well known, systematic methods for determining which data are more suitable for interpretation and analysis. During the drilling process, the drill string and BHA may undergo shock, vibration, torsional oscillations or whirl. Aside from the destructive nature of these modes of motion, data recorded during times when the drill string or BHA undergo such motion may be less reliable than when drilling is proceeding smoothly. It is desirable to have a method for discriminating data on the basis of drilling operating parameters and mode of motion such that data recorded under preferred drilling conditions may be selectively identified for analysis.